You can’t use the old AVR as a pre amp, because the pre-amp section is where all the HDMI and other “tech” resides. It’s the new AVR that has the updated stuff you wanted.
As
@Trebdp83 noted, if you feel the old 3805 had better amplifiers than the new 3700, then use the 3805 as a multichannel amplifier. Just connect the 3700’s pre-amp outputs to the 3805’s EXT In jacks, and connect your speakers to the 3805. Make sure ALL processing in the 3805 is bypassed – EQ, crossovers, delay, Night Cinema, etc.; you want the 3700 doing all that now.
FWIW, I’m currently doing the same thing in my system. My old AVR is a 65 lb. beast that can run 4-ohm speakers, but it doesn’t have HDMI. So I’m using it as an amplifier.
Regards,
Wayne A. Pflughaupt